A memory device is often produced using a semiconductor fabrication process. In the current application, the term “semiconductor” will be understood to mean any semiconductor material, including but not limited to bulk semiconductive materials (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). Moreover, it shall be understood that a semiconductor device may comprise conductive and insulative materials as well as a semiconductive material. The result of a semiconductor process may be a die comprising memory circuitry, and it may be desirable to test that circuitry at some point during the process of constructing a memory device comprising that die. For instance, testing may occur while the die is part of a semiconductor wafer, after singulation from the wafer, during die packaging, or once the memory device (chip) is completed.
One conventional method of testing such a chip is to have an external testing device write data to every memory cell of the chip, then read data from every memory cell, and compare the input with the output. Such a comparison may reveal cells that failed to store the data properly. The addresses corresponding to these defective cells can be stored by the external testing device, and that stored data may be used to repair the chip. In order to effect such repair, redundant cells are provided on the chip, as well as at least one bank of fuses or anti-fuses that controls access to the redundant cells. Assuming the bank to be comprised of anti-fuses, repair circuitry receives each address corresponding to a defective cell and, based on that address, blows at least one anti-fuse, thereby isolating the defective cell and associating the address with a redundant cell.
This error detect and repair scheme, however, raises issues. One such issue is the number of chips that may be tested at one time. A typical testing device is an AMBYX machine which can hold 256 chips. However, for cost reasons, the AMBYX has only 64 terminals (known as “DQ's” ) for reading from and writing to the chips. As a result, the chips must share these DQ resources. Assuming each chip has only four DQ's of its own (in which case the chips would be known as a “×4” part), then the AMBYX could access only 16 chips at one time. Thus a typical testing process would involve writing data to cells of 16 chips; reading data from cells of all 16 chips; comparing the written data with the read data; and, for cells wherein the written data and read data do not match, storing the addresses of those failed cells. These steps must be performed 15 more times in order to test all 256 chips on the AMBYX. Moreover, once repaired, the chips are often retested in a second test cycle to determine whether the repair was successful, thereby requiring even more time, especially if the chips must be removed from the AMBYX for repair and then placed back onto the AMBYX for retesting. Further, more than one type of test is often conducted. As a result, there is a desire in the art to shorten test time.